"Bright X-ray novae are so rare that they're essentially once-a-mission events and this is the first one Swift has seen," said Neil Gehrels, Swift principal investigator at NASA's Goddard Space Flight Center in Greenbelt, Md.
The rapidly brightening source triggered Swift's Burst Alert Telescope twice on the morning of Sept. 16 and once again the next day, NASA reported.
"This is really something we've been waiting for," Gehrels said.
An X-ray nova is a short-lived X-ray source that appears suddenly, reaches its emission peak in just a few days and then fades out over a period of months, started by a torrent of gas suddenly rushing toward one of the most compact objects known, either a neutron star or a black hole.
"The pattern we're seeing is observed in X-ray novae where the central object is a black hole," said Boris Sbarufatti, an astrophysicist at Brera Observatory in Milan, Italy, currently working with other Swift team members at Penn State.
"Once the X-rays fade away, we hope to measure its mass and confirm its black hole status."